Session 3: Maintenance of Road Drainage |
3.4 Maintenance Methods – Roadside Areas |
Including the shoulders and side slopes, most roadside area maintenance activities can be achieved by labour, and are suitable work for a mobile or local gang, or an individual lengthman living close to the road.
1. Shoulders
a. Defect: Obstructions such as rocks, trees or tree branches, soil heaps and abandoned vehicles/debris.
Main Causes
• Material fallen from slopes or trees, material washed onto the shoulders. • Debris left by road users.
If Neglected
• Hazard to road users.
• Obstruction of water flow from carriageway. Remedies
• Remove obstructions.
b. Defect: Shoulder Higher than Carriageway
Main Causes
• Carriageway surface material has collected on the shoulder by the action of traffic/water. • Soil from the cutting has slipped onto the shoulder.
• Vegetation has trapped material on the shoulder.
• Shoulder material has been displaced by the action of traffic. If Neglected
• Surface water can pond at the edge of the carriageway and weaken the pavement and shoulder. • Danger of accidents.
• Excess material may block the roadside ditch. Remedies
• Reshape or re-grade shoulder surface to the correct level. • Vegetation control.
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c. Defect: Shoulder Lower than Carriageway
d. Defect: Ruts or Depressions
Main Causes
• Traffic has been travelling on the shoulder and material has been worn away.
• Water erosion of the shoulder, settlement of the shoulder, the carriageway has been overlaid leaving the shoulder surface lower than the pavement.
If neglected
• Inadequate support for the road pavement.
• Water collects and softens the shoulder and pavement foundation.
• The edge of the pavement will break when vehicle wheels run over it, increasing the risk of accidents.
e. Defect: High Vegetation on Shoulders
Main Causes
Grass, weeds, bushes or trees have been allowed to grow unchecked. If neglected
• Surface water can pond at the edge of the carriageway and weaken the pavement.
• Silt accumulates at the edge of the carriageway, the visibility for road users is reduced, with increased risk of accidents with persons or animals.
• Increased fire hazard in the dry season. Remedies
• Vegetation control.
Fig: 19
Fig: 20
2. Drains
a. Defect: Obstructions
Main Causes
• Vegetation growth, bushes, fallen trees, debris, loose silt, loose rocks. If neglected
• Blockage of ditch. Remedies
• Clearing and cleaning.
b. Defect: Silting
Main Causes
• Invert slope is too flat; the water cannot flow at sufficient speed. If neglected
• Ditch blockage. Remedies
• Deepen ditch (de-silting), and/or provide new turnouts.
• Where deepening or turnouts are not possible because of topography, the construction of a new culvert with a drop-inlet may be possible, in order to discharge water onto the other side of the road.
c. Defect: Ponding in Drains and on Shoulders
Main Causes
• The ditch cross-section is too small; the ditch gradient is too flat. If neglected
• The shoulder material becomes soft and can easily erode. • The pavement can also be flooded and thereby weakened. Remedies
• Deepen ditch. • Provide new turnout.
d. Defect: Drain Cross-Section Destroyed (Unlined Drain)
Main Causes
• Vehicular or animal traffic; cave-in. If neglected
• Partial silting will result if the ditch sides have collapsed. • Erosion can start where water flow passes the blocked section.
Remedies
• Reshape/re-grade ditch, line drain.
e. Defect: Invert and Sides of Drains are Eroded
Main Causes
• Invert slope is too steep. If neglected
• The water flows at high speed and starts eroding the soil. The ditch becomes deeper (ravine). The sides then cave-in, the road shoulder and even part of the carriageway can be washed away.
Remedies Erosion control:
• Re-grade/realign drains. • Provide repair scour protection. • Line drain slopes and invert. • Construct cascade.
f. Defect: Drain Lining is Damaged
Main Causes
• Poor construction workmanship.
• Soil settlement, erosion of soil under ditch lining. • Poor alignment or sudden change in flow direction. If neglected
• When flowing water reaches the soil protected by the lining, erosion starts.
• The amount of soil washed away increases; the lining is further damaged by loss of support, leading to complete destruction of the lining.
Remedies Erosion control: • Repair lining. • Realign drain.
g. Defect at Drain Outfall
Main Causes • Flow too fast.
• Flow too concentrated for the soil at the outfall to resist. If neglected
• The erosion may eventually threaten the road as well as the surrounding land. Remedies
Reduce water flow and speed: • Realign drain to flatter gradient.
• Provide new turnout drain, upstream from existing. Reduce impact at outfall:
• Construct cascade. • Construct flow spreader. Erosion control for the soil: • Turfing.
• Wattling. • Stone pitching.
3. Manholes and Drainage Pipes
a. Defect: Water Overflowing at Manhole
Main Causes
• The manhole or connected underground pipes are blocked and water cannot flow as intended. If neglected
• Flooding of road shoulder or carriageway. • Drainage system becomes ineffective.
• Danger of earth slip or weakening of the pavement. Remedies
• Clear manhole and underground pipes.
b. Defect: Manhole Cover or Grating is Missing/Damaged
Main Causes
• Accident, vandalism.
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If neglected
• Open manholes become a danger to people and animals. Vegetation and debris have uncontrolled access and blockage can occur.
Remedies
• Replace manhole cover or grating.
c. Defect: Manhole is Covered with Soil and Vegetation
Main Causes
• Silting of the ground area at manhole; manhole cover level possibly set too low. If neglected
• Possible blockage of the drainage system at the manhole, due to an undetected accumulation of silt in the manhole. Remedies
• Clear manhole area. Public require education on:
• Not burning trash in drainage ways.
• Collecting leaves and other debris in drainage ways. • Cleaning of drainage ways.
4. Maintenance of Culverts and Bridges
a. Defect: Silting or Debris Blocking
Main Causes
• Invert slope too flat.
• Culvert constructed too low, so that material from the stream bed becomes deposited in the culvert. • Vegetation and floating debris carried by water has become lodged in the culvert.
If neglected
• The intended waterway opening will be reduced so that floodwater cannot flow. It will back-up or pond on the Fig: 23
upstream side of the culvert and may eventually overflow the road embankment. The road is then in danger of being washed away.
Remedies
• Clearing and cleaning.
• If floating debris is a problem, the provision of a debris rack should be considered.
b. Defect: Cracks
Main Causes
• Settlement of soil below culvert. If neglected
• Minor damage: If the settlement is minor, only light cracking will result in headwalls, wing walls and the main structure. This will hardly affect the functioning of the structure.
• Major damage: If the settlement is severe, it will cause large relative movement of culvert pipes so that embankment soil will enter through the cracks and block the culvert, or the culvert may collapse. The culvert must then be reconstructed.
Remedies • Repair cracks.
• Reconstruct at correct level and fall.
c. Defect: Erosion of Culvert Bed at Outlet
Main Causes
• The culvert invert has been constructed too steep so that the water flows too fast.
• The culvert invert has been constructed too flat with an excessive drop at the outfall (these are design or construction faults).
If neglected
• The streambed is washed. The culvert downstream head and wingwalls and even a section of the culvert and road embankment can collapse.
Remedies • Erosion repair.
• Construct outfall basin.
d. Defect: Minor Headwall Damage
Main Causes • Minor settlement. • Scour or erosion. If neglected
• Erosion at the headwall. • Culvert blockage or collapse. Remedies
• Headwall repair.